In two studies of Galapagos marine iguanas, one spanning 18 years and the other occupying eight years, scientists found that the herbivorous reptiles shrank as much as 2.7 inches, or 20 percent of their body length.

"Many people working with amphibians have seen this phenomenon but have not reported it because they don't believe it, or because the thinking is that vertebrates can't shrink," said Judy Stamps, a professor of evolution and ecology at the University of California at Davis.

The disbelievers included Martin Wikelski, a professor of ecology and evolution at the University of Illinois. Wikelski noted shrinkage in Galapagos marine iguanas over three periods  1982-83, 1987-88 and 1992-93  but dismissed the results as a measurement error. His thinking changed during El Niņo's biggest year, 1997-98.

"In 1997-98, the animals had shrunk too much to ignore," he said. "We thought this couldn't be an artifact, so we plotted out the data. It turned out to be very interesting."

The iguanas shrank to increase their chance of survival during El Niño.

Galapagos marine iguanas feed on algae along the tidal basins of the rocky shores of the remote islands. Normally, the islands experience cold, nutrient-rich currents from the west and south.
During El Niño, however, their usual diet disappears. "Green and red algal species, which are the marine iguana's preferred food ... are replaced in intertidal areas by brown algae which iguanas find hard to digest," according to the Nature article.

Upwards of 90 percent of the iguana population can die of starvation as a result of these environmental swings. Only the iguanas that shrink and slim down, making them more efficient at harvesting the tiny amounts of available food, survive.

"They shrink to reach a body size where survival is high. If they shrank a centimeter or so, they already increased their survival rate by 10 percent. If they shrink more, they can increase survivability by 35 percent," said Wikelski.

In subsequent La Niña years, when cold, nutrient-rich waters return to the Galapagos, the iguanas eat well, get fat and start growing longer again. Adult iguanas can shrink and grow repeatedly throughout their lifetime.

If researchers are able to figure out the mechanisms behind the shrinkage and the renewed growth in bone, they may be able to apply the same triggers to humans to treat diseases such as osteoporosis from aging or bone loss during space flight, said Stamps.

The researchers hypothesize that bone absorption accounts for much of the shrinkage.

"You have to remember that bone is living tissue that is constantly being replaced, just like muscle," said Stamps.